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Synthesis and growth of colloidal silica particles
Synthesis and Growth of Colloidal Silica Particles S. COENEN AND C. G. DE K R U I F Van't Hoff Laboratorium voor Fysische- en ColloMchemie, Padualaan 8, De Uithof, 3584 C H Utrecht, The Netherlands Received M a r c h 23, 1987; accepted July 20, 1987
In experimentalstudieson the staticand dynamicproperties of colloidalsuspensions,it is necessary to have particlestailoredwith regardto their size and/or opticalproperties. Startingwith a conventional St~Sbersilicaand a commercialLudoxsilica,we were able to growprimarypaniclesto a predetermined size. These panicles were subsequentlystericallystabilizedwith aliphatic stearyl chains. The resulting two colloidaldispersionsare relativelymonodisperse(_+11%)with overlappingsizedistributions. ©1988 AcademicPress, Inc. 1. I N T R O D U C T I O N
growth, it is difficult to control the size of the particles within a few nanometers; The reason In colloid physics there is a continuing need is that in the initial nucleation step it is the for specially prepared particle dispersions in number of nuclei that determines the ultimate order to perform experiments which provide size of the particles. In the StSber (2) synthesis, insight into the equilibrium and transport tetraethoxysilane (TES) is hydrolyzed in an properties of (concentrated) dispersions. In alkaline ethanol water mixture. After a hoorder to prepare the required dispersions, one mogeneous nucleation of (partially) hydrohas to tailor the particle properties. Size tai- lyzed TES, the silica particles grow by further loring is needed to prepare (mixtures of) par- hydrolysis of TES. Although this reaction ticles of a predetermined size. These particle pathway is generally accepted, details of the mixtures can be used to study sedimentation underlying reaction mechanisms have not behavior of silica particles in an environment been revealed. Nevertheless the pathway inof nonsedimenting latex particles or for the dicates that there is an almost trivial way of systematic study of ceramic compacts. By tailoring the particles to size, namely by adding changing the reaction conditions or adding TES to the reaction mixture after the nucle"dopants" one can change the density and ation stage. If the size of these relatively small particularly the optical density of the particles. nuclei is known (determined with either static This permits density tailoring and optical tai- or dynamic light scattering), then their number loring. Such particles may have a scattering can be calculated from the initial amount of nucleus (point scatterers) or a shell depending TES. Since additional TES will only increase on the details of the synthesis. These particles the volume of the particles, the particle size can be used to study the phenomenon of self can be controlled precisely. Both Jansen (3) and collective diffusion as described by Pusey and Philipse (4) explored these possibilities. et al. (1). One of the distinct cases of the theory The growth experiment as such was also peris the diffusion of optical tracer particles in a formed by Onada (5) whose sole purpose was "soup" of particles that are invisible but oth- to show its effectiveness. An advantage of diffusional growth is that erwise have the same properties. During the synthesis of colloidal particles polydispersity decreases with increasing size. by a process of nucleation and subsequent To exploit this, one needs appropriate seed 104 0021-9797/88 $3.00 Copyright© 1988 by AcademicPress, Inc. All fights of reproductionin any form reserved.
Journal of Colloidand InterfaceScience, Vol. 124, No. 1, July 1988
SILICA PARTICLES
particles. In Chapter VI of his thesis (3), Jansen describes experiments in which he grew Strber silica particles. Alternative sources of seed particles are the commercially available silica particles (for a review see Ref. (8)). These particles are inexpensive, small, relatively monodisperse, and, even more interesting, have a higher optical and mass density than silica prepared by the Strber synthesis. Jansen also experimented with commercial silica, where he provided the particles with a Strber environment (i.e., alcohol, water, and NH3) in which to grow. The purpose of this paper is to give details of the experimental conditions during the conceptually simple but experimentally tricky synthesis since in many cases the colloidal systems flocculate or gel. If one tries to grow silica particles smaller than 30 nm one frequently ends up with a gel, presumably due to the fast hydrolysis of the TES. We shall describe conditions for growing both commercial Ludox (6) particles and Strber silica particles to a predetermined size. Although the Strber synthesis has been described before (2) we outline it again briefly. We sterically stabilized the particles by grafting stearyl alcohol onto the surface silanol groups (7). 2. PREPARATIONS
2.1. Strber Synthesis and Growth of Silica Particles Three thousand milliliters of freshly distilled ethanol was placed in a well-cleaned flask. One hundred-forty milliliters ammonia (25%) and 150 ml freshly vacuum distilled tetraethoxysilane was added while stirring. The mixture was then stirred gently for 24 h. In this (particular) case the particles (denoted SC9.0) formed were found to have a radius of 20 n m as determined with dynamic light scattering. The slow addition of (as calculated) 150 ml TES yielded particles with a radius of 25 nm. These colloidal particles are denoted as SC9.1.
2.2. Growth of Ludox Silica Particles Having looked closely at the accounts of Jansen's experiments on the Ludox particles,
105
we decided that it would be more straightforward to start with an ammonia stabilized Ludox instead of trying to dialyze both the water and the sodium ions. After several attempts, we finally succeeded in diluting the Ludox dispersion and were able to initiate a growth stage. The procedure was as follows: We put 500 ml doubly distilled water, 10 ml ammonia (25%), and 150 ml Ludox AS-40% into a wellcleaned flask and stirred the mixture thoroughly. We then diluted this dispersion by adding approximately 9 liters freshly distilled alcohol and two 50-ml quantities of ammonia (25%). This gave a bluish dispersion with a weight concentration of 0.008 g/ml. The particle radius of the Ludox was found to be 18 nm (these particles are denoted SLC4.0). Assuming a density of 2.2 g/ml for the Ludox and 1.8 g/ml for the silica, we calculated that 410 ml TES needed to be added to grow the Ludox particles to a radius of 25 nm. The resuiting system is denoted as SLC4.1.
2.3. Steric Stabilization Van Helden (7) developed a method based on Iler's synthesis (8) in which the initially charge stabilized silica particles are covered with a layer of aliphatic chains and thus become sterically stabilized and lyophilic. Therefore they are readily dispersible in an organic solvent such as cyclohexane or toluene. Following Van Helden's procedure we disTABLE I Compilation of Experimental Results Characterizing Colloidal Silica Particles Radius from DLS (nm)
Radius from TEM (tam)
Width of distribution
SC9.0 SC9.1 SC9
20.9 + 1 26.3 + 1 33.9 ± 1
-19 19
-0.10 0.13
SLC4.0 (Ludox) SLC4.1 SLC4 1:1 mixture SC9/SLC4
18.2 ± 1 24.8 + 1 30.8 + 1
15 -21
0.10 -0.10
19
0.11
Journal of Colloid and Interface Science, Vol. 124, No. 1, July 1988
106
COENEN
Journal of Colloid and Interface Science, Vol. 124, No. 1, July 1988
AND
DE KRUIF
107
SILICA PARTICLES
FIG. 1. (a) TEM recording of silica particles denoted as SC9.0. (b) TEM of silica SC9.1. (c) TEM of silica coated with octadecyl chains.
solved a quantity ofstearyl alcohol (amounting to twice the weight of the TES used) in three times its own volume of warm ethanol in a round-bottom flask. The previously made alcosol was added in portions, and the ammonia and some of the alcohol/water were distilled off after each addition. T o ensure that the water is distilled off first, one can add a few portions of pure ethanol or better still propanol. After the alcohol was removed the stearyl alcohol was heated and partially vacuum distilled with a stream of N2 at 180°C. The excess stearyl alcohol containing the silica particles was cooled to room temperature. Then the contents of the flask were dissolved in cyclohexane. The stearyl alcohol was removed by repeated sedimentation in a centrifuge and redispersion in cyclohexane. The resulting transparent dispersions obtained from the alcosols SC9.1 and SLC4.1 are denoted as SC9 and SLC4, respectively, with the L indicating the Ludox core.
2.4. Calculations
The total volume of silica resulting from the Strber synthesis is
vs=
b~ M(SiO2)p(TES)
M(TES)0(silica) '
where bl is starting volume of TES in milliliters, o is density in grams per milliliter, and M is molar mass; o(TES) = 0.933 g/ml; M(SiO2) = 60.08 g/mole; M(TES) = 208.33 g/mole; p(silica) = 1.8 g/ml; and 0(ludox) = 2.2 g/ml. Depending on the number of nuclei, this will result in an average particle radius rl (to be measured). Growing particles to r2 will require the addition of an extra volume of[(r2/rl) 3 -~ 1]b~ of TES. In order to grow the Ludox particles, one first calculates the number of particles per milliliter (nl(1/ml)) from the weight concentration (wl(g/ml)), the density of Ludox particles (2.2 g/mole), and the particle radius rl(cm) according to Journal of Colloid and Interface Science, Vol. 124, No. 1, July 1988
108
COENEN AND DE KRUIF
I~IG. 2. (a) TEM of ludox particles SLC4.0. (b) TEM of SLC4 coated with octadecyl chains. Journal of Colloid and Interface Science, Vol. 124, No. 1, July 1988
SILICA PARTICLES
109
FIG. 3. TEM of mixture of SC9 and SCL4.
/71 =
wl 1 2.2 4/3rrr 3"
The volume b2 of TES to be added per milliliter of alcosol then follows from bz = W~R/(Mo(silica)),
where R = r2/rl 3 3 - 1 and M = {M(SiO2) X p(TES)}/{M(TES)p(silica)}. 3. RESULTS AND CONCLUSIONS
We shall now summarize the details of the particle characterizations revealed by TEM and dynamic light scattering (DLS). From Table I it can be seen that the mean particle radius can be controlled precisely. The difference between the TEM and dynamic light scattering radius stems from the fact that different properties are measured and the TEM magnification is somewhat uncertain. Moreover results depend on the electron beam intensity. More
trustworthy than the absolute size is the size distribution determined from TEM micrographs expressed here as the standard deviation from the mean. Typical micrographs for SC9 and SLC4 are shown in Figs. 1 and 2 while Fig. 3 shows the mixture of SC9 and SLC4. When dynamic light scattering is done on dilute suspensions, the presence of clusters or polydispersity is revealed by the scattering angle dependence of the diffusion coefficient. We therefore plot in Fig. 4 for both SC9 and SLC4 the diffusion coefficient as a function of K 2 where K = 4~rn/~ sin 0/2 with n the index of refraction of cyclohexane (1.425) and X the wavelength of the laser light used (~ = 488 nm). These plots clearly show the absence of a high degree of polydispersity (or clusters) in the final product, i.e., a lyophilic colloid in cyclohexane. In this study we have concentrated on the synthetic aspects and have given less attention to the precise tailoring of the particles. NevJournal of ColloM and Interface Science, Vol. 124,No. 1, July 1988
110
COENEN AND DE KRUIF
~81
(o)
aspect requires m o r e d e t a i l e d study a n d is outside the scope o f this paper. ACKNOWLEDGMENTS
TT
~b> >
i0InK 2] m -2
FIG. 4. Diffusion coefficient of SC9 (a) and SC£A (b) cyclohexane as a function o f K 2 as measured with dynamic light scattering. Note vertical axis is offset. The calculated
Stokes radii are given in Table I.
ertheless results show that the size distributions o f the two particle b a t c h e s SC9 a n d SLC4 alm o s t coincide. T h u s we are confident t h a t the problem of preparation and characterization is n o longer with the synthesis b u t is m u c h m o r e with the e x p e r i m e n t s to define a n d m e a sure the particle size a n d size distribution. This
Journal of Colloid and Interface Science, Vol. 124, No. 1, July 1988
We thank Dr. Jan Willem Jansen and Dr. Albert Philipse for their suggestions and remarks and Dr. Aernout van Veluwen for doing some of the light scattering measurements. The stimulating interest of Professor A. Vrij in the subject is very much appreciated. REFERENCES 1. Pusey, P. N., Fijnaut, H. M., and Vrij, A., J. Chem. Phys. 77, 4270 (1982). 2. StSber, W., Fink, A., and Bohn, E., J. Colloid Interface Sci. 26, 62 (1968). 3. Jansen, J. W., "Attractive Interactions in Sterically Stabilized Silica Dispersions," Thesis, Utrecht, 1985. 4. Philipse, A., Internal report No. 54, February 1985 Utrecht. 5. Onada, G. J., and Liniger, E. G., Abstracts 5th Int. Conf. Surf. Coll. Sci. Potsdam, June 24-28, Potsdam, NJ, 1985. 6. Ludox, DuPont, Wilmington, DE. 7. Van Helden, A. K., Jansen, J. W., and Vrij, A., J. Colloid Interface Sci. 81, 354 (1984). 8. Iler, R. K., U.S. Patent 2,801,185 (1957).
In experimentalstudieson the staticand dynamicproperties of colloidalsuspensions,it is necessary to have particlestailoredwith regardto their size and/or opticalproperties. Startingwith a conventional St~Sbersilicaand a commercialLudoxsilica,we were able to growprimarypaniclesto a predetermined size. These panicles were subsequentlystericallystabilizedwith aliphatic stearyl chains. The resulting two colloidaldispersionsare relativelymonodisperse(_+11%)with overlappingsizedistributions. ©1988 AcademicPress, Inc. 1. I N T R O D U C T I O N
growth, it is difficult to control the size of the particles within a few nanometers; The reason In colloid physics there is a continuing need is that in the initial nucleation step it is the for specially prepared particle dispersions in number of nuclei that determines the ultimate order to perform experiments which provide size of the particles. In the StSber (2) synthesis, insight into the equilibrium and transport tetraethoxysilane (TES) is hydrolyzed in an properties of (concentrated) dispersions. In alkaline ethanol water mixture. After a hoorder to prepare the required dispersions, one mogeneous nucleation of (partially) hydrohas to tailor the particle properties. Size tai- lyzed TES, the silica particles grow by further loring is needed to prepare (mixtures of) par- hydrolysis of TES. Although this reaction ticles of a predetermined size. These particle pathway is generally accepted, details of the mixtures can be used to study sedimentation underlying reaction mechanisms have not behavior of silica particles in an environment been revealed. Nevertheless the pathway inof nonsedimenting latex particles or for the dicates that there is an almost trivial way of systematic study of ceramic compacts. By tailoring the particles to size, namely by adding changing the reaction conditions or adding TES to the reaction mixture after the nucle"dopants" one can change the density and ation stage. If the size of these relatively small particularly the optical density of the particles. nuclei is known (determined with either static This permits density tailoring and optical tai- or dynamic light scattering), then their number loring. Such particles may have a scattering can be calculated from the initial amount of nucleus (point scatterers) or a shell depending TES. Since additional TES will only increase on the details of the synthesis. These particles the volume of the particles, the particle size can be used to study the phenomenon of self can be controlled precisely. Both Jansen (3) and collective diffusion as described by Pusey and Philipse (4) explored these possibilities. et al. (1). One of the distinct cases of the theory The growth experiment as such was also peris the diffusion of optical tracer particles in a formed by Onada (5) whose sole purpose was "soup" of particles that are invisible but oth- to show its effectiveness. An advantage of diffusional growth is that erwise have the same properties. During the synthesis of colloidal particles polydispersity decreases with increasing size. by a process of nucleation and subsequent To exploit this, one needs appropriate seed 104 0021-9797/88 $3.00 Copyright© 1988 by AcademicPress, Inc. All fights of reproductionin any form reserved.
Journal of Colloidand InterfaceScience, Vol. 124, No. 1, July 1988
SILICA PARTICLES
particles. In Chapter VI of his thesis (3), Jansen describes experiments in which he grew Strber silica particles. Alternative sources of seed particles are the commercially available silica particles (for a review see Ref. (8)). These particles are inexpensive, small, relatively monodisperse, and, even more interesting, have a higher optical and mass density than silica prepared by the Strber synthesis. Jansen also experimented with commercial silica, where he provided the particles with a Strber environment (i.e., alcohol, water, and NH3) in which to grow. The purpose of this paper is to give details of the experimental conditions during the conceptually simple but experimentally tricky synthesis since in many cases the colloidal systems flocculate or gel. If one tries to grow silica particles smaller than 30 nm one frequently ends up with a gel, presumably due to the fast hydrolysis of the TES. We shall describe conditions for growing both commercial Ludox (6) particles and Strber silica particles to a predetermined size. Although the Strber synthesis has been described before (2) we outline it again briefly. We sterically stabilized the particles by grafting stearyl alcohol onto the surface silanol groups (7). 2. PREPARATIONS
2.1. Strber Synthesis and Growth of Silica Particles Three thousand milliliters of freshly distilled ethanol was placed in a well-cleaned flask. One hundred-forty milliliters ammonia (25%) and 150 ml freshly vacuum distilled tetraethoxysilane was added while stirring. The mixture was then stirred gently for 24 h. In this (particular) case the particles (denoted SC9.0) formed were found to have a radius of 20 n m as determined with dynamic light scattering. The slow addition of (as calculated) 150 ml TES yielded particles with a radius of 25 nm. These colloidal particles are denoted as SC9.1.
2.2. Growth of Ludox Silica Particles Having looked closely at the accounts of Jansen's experiments on the Ludox particles,
105
we decided that it would be more straightforward to start with an ammonia stabilized Ludox instead of trying to dialyze both the water and the sodium ions. After several attempts, we finally succeeded in diluting the Ludox dispersion and were able to initiate a growth stage. The procedure was as follows: We put 500 ml doubly distilled water, 10 ml ammonia (25%), and 150 ml Ludox AS-40% into a wellcleaned flask and stirred the mixture thoroughly. We then diluted this dispersion by adding approximately 9 liters freshly distilled alcohol and two 50-ml quantities of ammonia (25%). This gave a bluish dispersion with a weight concentration of 0.008 g/ml. The particle radius of the Ludox was found to be 18 nm (these particles are denoted SLC4.0). Assuming a density of 2.2 g/ml for the Ludox and 1.8 g/ml for the silica, we calculated that 410 ml TES needed to be added to grow the Ludox particles to a radius of 25 nm. The resuiting system is denoted as SLC4.1.
2.3. Steric Stabilization Van Helden (7) developed a method based on Iler's synthesis (8) in which the initially charge stabilized silica particles are covered with a layer of aliphatic chains and thus become sterically stabilized and lyophilic. Therefore they are readily dispersible in an organic solvent such as cyclohexane or toluene. Following Van Helden's procedure we disTABLE I Compilation of Experimental Results Characterizing Colloidal Silica Particles Radius from DLS (nm)
Radius from TEM (tam)
Width of distribution
SC9.0 SC9.1 SC9
20.9 + 1 26.3 + 1 33.9 ± 1
-19 19
-0.10 0.13
SLC4.0 (Ludox) SLC4.1 SLC4 1:1 mixture SC9/SLC4
18.2 ± 1 24.8 + 1 30.8 + 1
15 -21
0.10 -0.10
19
0.11
Journal of Colloid and Interface Science, Vol. 124, No. 1, July 1988
106
COENEN
Journal of Colloid and Interface Science, Vol. 124, No. 1, July 1988
AND
DE KRUIF
107
SILICA PARTICLES
FIG. 1. (a) TEM recording of silica particles denoted as SC9.0. (b) TEM of silica SC9.1. (c) TEM of silica coated with octadecyl chains.
solved a quantity ofstearyl alcohol (amounting to twice the weight of the TES used) in three times its own volume of warm ethanol in a round-bottom flask. The previously made alcosol was added in portions, and the ammonia and some of the alcohol/water were distilled off after each addition. T o ensure that the water is distilled off first, one can add a few portions of pure ethanol or better still propanol. After the alcohol was removed the stearyl alcohol was heated and partially vacuum distilled with a stream of N2 at 180°C. The excess stearyl alcohol containing the silica particles was cooled to room temperature. Then the contents of the flask were dissolved in cyclohexane. The stearyl alcohol was removed by repeated sedimentation in a centrifuge and redispersion in cyclohexane. The resulting transparent dispersions obtained from the alcosols SC9.1 and SLC4.1 are denoted as SC9 and SLC4, respectively, with the L indicating the Ludox core.
2.4. Calculations
The total volume of silica resulting from the Strber synthesis is
vs=
b~ M(SiO2)p(TES)
M(TES)0(silica) '
where bl is starting volume of TES in milliliters, o is density in grams per milliliter, and M is molar mass; o(TES) = 0.933 g/ml; M(SiO2) = 60.08 g/mole; M(TES) = 208.33 g/mole; p(silica) = 1.8 g/ml; and 0(ludox) = 2.2 g/ml. Depending on the number of nuclei, this will result in an average particle radius rl (to be measured). Growing particles to r2 will require the addition of an extra volume of[(r2/rl) 3 -~ 1]b~ of TES. In order to grow the Ludox particles, one first calculates the number of particles per milliliter (nl(1/ml)) from the weight concentration (wl(g/ml)), the density of Ludox particles (2.2 g/mole), and the particle radius rl(cm) according to Journal of Colloid and Interface Science, Vol. 124, No. 1, July 1988
108
COENEN AND DE KRUIF
I~IG. 2. (a) TEM of ludox particles SLC4.0. (b) TEM of SLC4 coated with octadecyl chains. Journal of Colloid and Interface Science, Vol. 124, No. 1, July 1988
SILICA PARTICLES
109
FIG. 3. TEM of mixture of SC9 and SCL4.
/71 =
wl 1 2.2 4/3rrr 3"
The volume b2 of TES to be added per milliliter of alcosol then follows from bz = W~R/(Mo(silica)),
where R = r2/rl 3 3 - 1 and M = {M(SiO2) X p(TES)}/{M(TES)p(silica)}. 3. RESULTS AND CONCLUSIONS
We shall now summarize the details of the particle characterizations revealed by TEM and dynamic light scattering (DLS). From Table I it can be seen that the mean particle radius can be controlled precisely. The difference between the TEM and dynamic light scattering radius stems from the fact that different properties are measured and the TEM magnification is somewhat uncertain. Moreover results depend on the electron beam intensity. More
trustworthy than the absolute size is the size distribution determined from TEM micrographs expressed here as the standard deviation from the mean. Typical micrographs for SC9 and SLC4 are shown in Figs. 1 and 2 while Fig. 3 shows the mixture of SC9 and SLC4. When dynamic light scattering is done on dilute suspensions, the presence of clusters or polydispersity is revealed by the scattering angle dependence of the diffusion coefficient. We therefore plot in Fig. 4 for both SC9 and SLC4 the diffusion coefficient as a function of K 2 where K = 4~rn/~ sin 0/2 with n the index of refraction of cyclohexane (1.425) and X the wavelength of the laser light used (~ = 488 nm). These plots clearly show the absence of a high degree of polydispersity (or clusters) in the final product, i.e., a lyophilic colloid in cyclohexane. In this study we have concentrated on the synthetic aspects and have given less attention to the precise tailoring of the particles. NevJournal of ColloM and Interface Science, Vol. 124,No. 1, July 1988
110
COENEN AND DE KRUIF
~81
(o)
aspect requires m o r e d e t a i l e d study a n d is outside the scope o f this paper. ACKNOWLEDGMENTS
TT
~b> >
i0InK 2] m -2
FIG. 4. Diffusion coefficient of SC9 (a) and SC£A (b) cyclohexane as a function o f K 2 as measured with dynamic light scattering. Note vertical axis is offset. The calculated
Stokes radii are given in Table I.
ertheless results show that the size distributions o f the two particle b a t c h e s SC9 a n d SLC4 alm o s t coincide. T h u s we are confident t h a t the problem of preparation and characterization is n o longer with the synthesis b u t is m u c h m o r e with the e x p e r i m e n t s to define a n d m e a sure the particle size a n d size distribution. This
Journal of Colloid and Interface Science, Vol. 124, No. 1, July 1988
We thank Dr. Jan Willem Jansen and Dr. Albert Philipse for their suggestions and remarks and Dr. Aernout van Veluwen for doing some of the light scattering measurements. The stimulating interest of Professor A. Vrij in the subject is very much appreciated. REFERENCES 1. Pusey, P. N., Fijnaut, H. M., and Vrij, A., J. Chem. Phys. 77, 4270 (1982). 2. StSber, W., Fink, A., and Bohn, E., J. Colloid Interface Sci. 26, 62 (1968). 3. Jansen, J. W., "Attractive Interactions in Sterically Stabilized Silica Dispersions," Thesis, Utrecht, 1985. 4. Philipse, A., Internal report No. 54, February 1985 Utrecht. 5. Onada, G. J., and Liniger, E. G., Abstracts 5th Int. Conf. Surf. Coll. Sci. Potsdam, June 24-28, Potsdam, NJ, 1985. 6. Ludox, DuPont, Wilmington, DE. 7. Van Helden, A. K., Jansen, J. W., and Vrij, A., J. Colloid Interface Sci. 81, 354 (1984). 8. Iler, R. K., U.S. Patent 2,801,185 (1957).